The properties of lignocellulosic materials, such as paper, linerboard, corrugated (also called fluted) medium, cartonboard, and paper structures in general, can be altered to varying degrees by incorporating additives into their matrices or webs. For example, it is known that the strength of linerboard in compression or tension can be substantially increased by incorporating sodium silicate or starch into the structure of the linerboard. However, such incorporation of additives usually affects other functional properties of the lignocellulosic materials adversely. One important functional property of the lignocellulosic materials which may be adversely affected is adhesive bondability. This is especially important if the treated lignocellulosic materials are to be combined with other treated or untreated lignocellulosic materials in a secondary process such as an adhesive bonding operation. One example of an adhesive bonding operation is the formation of corrugated paper structures in which a treated linerboard needs to be combined and adhered to a fluted medium in a corrugator using typical operating conditions, such as speed, temperature profile, etc., and typical corrugating adhesives.
Usually the additives are in the form of a solution or dispersion, and for the purpose of this disclosure the terms solution and dispersion are used interchangeably. Similarly, for the purpose of this disclosure, the terms additive, agent, active, and saturant are used interchangeably. Finally, the terms incorporation, treatment, impregnation, and saturation are used interchangeably for the purpose of this disclosure.
One means of incorporating additives into lignocellulosic materials is by coating one or both surfaces of these materials with the additives. Typical coating operations include spraying, rod coating, blade coating, gravure coating, etc. However, coating operations typically do not achieve sufficient incorporation of the additives inside the lignocellulosic materials which, in most cases, results in minimal property changes. For example, coating of lignocellulosic materials with strengthening agents typically results in minimal strength improvements since most of the strengthening agents remain localized near the incorporation surfaces. In addition to minimal property changes, coating of the surface or surfaces of lignocellulosic materials often causes difficulties when the treated surface or surfaces are to be adhesively bonded to other materials, for example, to another lignocellulosic material. These difficulties arise from the fact that the coating typically covers the surface or surfaces with additives, thereby eliminating adhesive bonding sites, and also limiting or preventing adhesive penetration into the lignocellulosic material.
One approach that addresses this bonding problem is described in U.S. Pat. No. 5,698,295, issued to Benner et al., and assigned to Dallas Enviro-Tek International, Inc. In this patent, a specialty cold set adhesive, which is often more expensive than a conventional starch adhesive, is used to adhesively bond the coated linerboard with coated medium. Another method that addresses the bonding problem is to reduce the speed of the bonding equipment. In the case that this bonding process is the corrugating process, it has been observed that the corrugating singlefacer must be slowed to an operating rate of less than 300 feet per minute (300 fpm; 91.4 m/min; 1.52 m/s). Commercial corrugating operations typically run in excess of 600 fpm (3.05 m/s). Both approaches described above may result in high process costs, or high material costs, or both.
Another method of incorporating additives into lignocellulosic materials is full impregnation or saturation of these materials. This can be carried out via immersion of the lignocellulosic materials into a bath containing a solution or dispersion of additives for a sufficient period of time. However, such an approach does not solve the adhesive bondability problem, since immersion into the bath would still cause the additives to coat both surfaces of the lignocellulosic materials.
An approach that attempts to solve both problems of insufficient incorporation of additives and lack of adhesive bondability in papers such as linerboard is described in U.S. Pat. No. 5,776,546, issued to Long, and assigned to MIPLY Equipment Inc. The MIPLY process uses one or two converging pressure chambers (e.g. in the form of a journal bearing) to achieve paper web impregnation with various additives. Furthermore, the '546 patent describes pushing the additives away from the impregnation surface with a second additive to displace the first additive from the surface, such that the impregnation surface can be made more suitable for adhesive bonding. However, the process described in the '546 patent requires the use of a secondary treatment and additional processing steps resulting in additional costs and complexity.
What has been missing is a simple and financially-viable means of producing partially impregnated lignocellulosic materials providing increased strength or other enhanced material properties while at the same time also maintaining sufficient adhesive bondability.